Hybrid circuit with electron valve separating element



May 19, 1970 o. LINDGREN ET AL 3,5

HYBRID CIRCUIT WITH ELECTRON VALVE SEPARATING ELEMENT 2 Sheets-Sheet 1 Filed Nov. 9, 1966 INVENIOR. OWE LINDGRE/V GUN/VAR KJELLANDER B LENA/ART .SKOOG May 19, 1970 o. LINDGREN ET AL 3,513,259

HYBRID CIRCUIT WITH ELECTRON VALVE SEPARATING ELEMENT Filed NOV. 9, 1966 2 Sheets-Sheet 2 INVEN'I'OR. OWE L/NDGREN ATTORNEYS United States Patent 3,513,259 HYBRID CIRCUIT WITH ELECTRON VALVE SEPARATING ELEMENT Owe Lindgren, Farsta, Gunnar Kjellander, Hagersten, and Lennart Skoog, Farsta, Sweden, assignors to Aktiebolaget Gylling & Co., Stockholm, Sweden, a corporation of Sweden Filed Nov. 9, 1966, Ser. No. 593,188 Claims priority, application Sweden, Dec. 16, 1965, 16,307/65 Int. Cl. H04m 9/08 US. Cl. 179-1 10 Claims ABSTRACT OF THE DISCLOSURE A hybrid circuit connecting a two-conductor signal transmission line to a transmitting device and to a receiving device. The hybrid circuit includes an electron valve, e.g., a transistor, having three electrodes, and has circuits (a) for applying first signal voltages developed by said transmitting device to the base and emitter electrodes with substantially equal phase and equal amplitude and (b) for supplying the second signal voltages originating from the transmission line to the base and emitter electrodes with greater amplitudes than the first signal voltages. The electron voltage valve providing an output signal at the collector electrode controlled by the voltage applied between said base and emitter electrodes and a coupling circuit enables applying the output signal to the receiving device.

In, for example, a loudspeaking telephone instrument, a hybrid circuit is provided for connecting the microphone of the instrument and the loudspeaker of the instrument to a two-conductor line, to which the instrument is connected. The object of the hybrid circuit is to let signals which arrive from the line to pass to the loudspeaker and to let signals which originate from the microphone to pass to the line but to prevent signals which originate from the microphone to pass to the loudspeaker of the instrument. Hereinafter, whenever the term fork circuit is used, it means a hybrid circuit.

Fork circuits of this kind are already known in a great number of different embodiments. All known fork circuits suffer from the drawback, that their function is very much dependent on a balancing network included in the fork circuit. The impedance value of such balancing network, with respect to its phase angle as well as to its magnitude, should be carefully correlated to corresponding quantities of the impedance of the line. However the impedance of the line may be different for different cases, and it will also be subject to variations, therefore, the function of the fork circuit is not reliable.

The present invention relates to a fork circuit for connecting a two-conductor communication line to a transmitting device, for example, to a microphone, and to a receiving device, for example, to a sound reproducer, in which great variations with respect to the impedance of the connected line may be allowed without jeopardizing the function of the fork circuit.

The invention is substantially characterized by the fact that the fork circuit comprises an electron valve with three electrodes, said electron valve being so designed, that the voltage or the current which i supplied between two of its electrodes results in a voltage or current between the third electrode and one of the first mentioned electrodes, means being provided for supplying voltage from the transmitting device to both of the first mentioned electrodes with substantially equal phase value and equal amplitude value, and supplying voltage coming ice from the line with a greater amplitude to one of the first mentioned electrodes than to the other of said electrodes, and means for providing from the third electrode a voltage or a current to a receiving device.

In the following, the invention will be described with reference to the accompanying drawings, which diagrammatically illustrate a fork circuit according to the invention.

FIGS. 1 and 2 illustrate two embodiments of a hybrid circuit in accord with the present invention.

In the drawing, L1 and L2 represent two parts of a two-conductor line, to which the fork circuit is connected. T he line L1, L2 is, by means of a Graetz rectifier bridge G, connected to a ground conductor i.e. to a conductor having substantially zero potential, which is designated 0, and also to one end of a voltage divider, which comprises three impedances Z1, Z2 and Z3. The other end of this voltage divider is connected to the ground conductor 0 through a first storage condenser C1.

A microphone M is connected to a base electrode in a transistor MT through an amplifier MF and through a conductor ML, the collector electrode of said transistor MT being galvanically connected to the interconnection point between the impedances Z1 and Z2.

The emitter electrode of the transistor MT is, through a resistor R4, connected to one terminal of a second storage condenser C2, the other terminal of which is connected to the conductor 0. The storage condenser C2 is connected in parallel with a voltage stabilizer, for example a Zener diode DZ, in order to stabilize the voltage to which the storage condenser C2 is charged.

The storage condenser C1 is charged through the voltage divider Z1, Z2, Z3, and the storage condenser C2 is charged through the impedance Z1 and through the transistor MT and through a resistor R4. The storage condenser C2 has much greater capacitance than the storage condenser C1.

The fork circuit comprises a transistor GT, the base electrode of which is connected, through a capacitor, to the interconnection point between the rectifier bridge G and the impedance Z1. The emitter electrode of the transistor GT is, through a resistor R5 and a capacitor, connected to the interconnection point between the impedances Z2 and Z3. Further, said emitter electrode is connected to the conductor 0 through a resistor R6.

From the collector electrode of the transistor GT, a signal voltage is taken out through a capacitor, and through a conductor HL to an output amplifier HF, /which supplies the loudspeaker H with signal energy.

The base electrode of the transistor MT is supplied with a suitable bias voltage from the first storage condenser Cl, through a resistor R7, and the base electrode of the transistor GT is supplied with bias voltage from a voltage divider, which comprises two resistors R8 and R9, said voltage divider being connected between the second storage condenser C2 and the conductor 0. The collector electrode of the transistor GT is connected to the first storage condenser C1, through a resistor R10.

The transistor MT serves as an amplifying means for the signals which originate from the microphone M and signals which are supplied to the line L1, L2, and also as a means, which conducts DC-current from the line L1, L2 to the storage condenser CI, but at the same time prevents the signals which appear on the line L1, L2 from reaching the devices which are supplied With DC-current from the storage condenser C2.

The signals, which originate from the microphone M, and which are amplified in the amplifier MP and in the transistor MT, are supplied to the base electrode of the transistor GT and also to the emitter electrode of said transistor. The impedances Z1 and Z2 are so adjusted with respect to each other and to the other impedances in the device and in the line L1, L2, that the base electrode and the emitter electrode of the transistor GT are supplied with signal voltages of substantially the same amplitude and the same phase. The result of this will be, that said signals will not appear on the collector electrode of the transistor GT and, therefore, they will not be reproduced by the loudspeaker H. If, on the contrary, a signal arrives from the line L1, L2, it will be supplied to the base electrode in the transistor GT at a higher amplitude than to the emitter electrode of the same transistor. The result of this will be that such a signal is amplified by the transistor GT and is supplied through the amplifier HF to the loudspeaker H.

The foregoing occurs under the condition that the impedances Z1 and Z2 are suitably adjusted with respect to each other and to the impedance Z3. In a preferred embodiment of the invention, the impedance Z1 has the value of 15 ohms and the impedance Z2 the value of 10 ohms. The impedances Z1 and Z2 may be totally resistive, and they should, in each case, be galvanically conducting. The impedance Z3 is constituted by a resistor R3, which may have a value of about 680 ohms, and an RC-circuit, which is connected in parallel with said resistor R3, consists of a capacitance C6 and a resistor R11 connected in series with said capacitance. (The value of resistor R6 is great as compared with that of the resistor R3, and, therefore, its influence on the value of the impedance Z3 is negligible.) The value of the impedance Z3 in respect of its magnitude and its phase angle should be related to the magnitude and the phase angle of the impedance of the line L1, L2, as the ratio between the impedance Z2 and the impedance Z1.

The impedance which the device has, as seen from the connected line L1, L2, is substantially determined by the voltage divider Z1, Z2, Z3, because the transistor GT has high impedance as seen from its base electrode and the transistor MT has a very high impedance as seen from its collector electrode, and the resistors R5 and R6 are of high values as compared with the impedances Z1, Z2, Z3.

In the described embodiment, the impedances Z1 and Z2 are resistive and the impedance Z3 is complex. It the line L1, L2 has a mainly resistive impedance, the impedance Z3 may also be resistive. On the other hand, it may in some cases, be desirable to make the impedances Z1 and Z2 with complex impedance values.

As seen from the connected telephone instrument, the line L1, L2 has, in most cases, a complex impedance, which is substantially resistive and capacitive for the frequencies which are used. Therefore, the impedance Z3 is designed to be resistive and capacitive. The microphone amplifier MP and the circuit components belonging to the transistor MT are so designed, that the correct frequency characteristic will be obtained, when the capactive load, which results from the line and from the impedance Z3, is active in the collector circuit of the transistor MT.

The capacitive load to the line caused by the fork circuit, is, however, often a disadvantage. The fork circuits hitherto used usually include a transformer, one winding of which is connected to the line. This means, that the line in these cases is loaded by a complex impedance which for the used frequencies is substantially resistive and inductive. The telephone exchange stations are usually designed in consideration of this fact and, therefore, a more universal use of telephone instruments with a capacitive input impedance would possibly cause difficulties.

It is, however, possible to eliminate that disadvantage to a certain degree by connecting an impedance Z4 in parallel to the impedances Z2 and Z3. The impedance Z4 should have a suitable value. Such an impedance is shown by dotted lines in FIG. 1. Said impedance Z4 is directly included in that complex of impedances to which the line L1, L2 is connected, and, therefore, it is possible to adjust the impedance value in a way which is most favorable from that point of view. In order that the line should not be loaded by a lower impedance than what is suitable or what is prescribed in order that the instrument should be allowed to be connected to a general telephone line, it is preferable to choose a higher impedance value for the impedance Z3 (for example, by choosing a higher value for the resistor R3) when the impedance Z4 is present. The impedance Z4 does not alter the balance of the fork circuit, but it is included in that system of impedances, which form a load to the collector of the transistor MT. Because it is desirable for the sake of frequency characteristic that this system of impedances, as seen from the collector, is resistive and capacitive, it is preferable not to choose an inductive phase angle for the impedance Z4. In practice, it has been proved that the impedance Z4 preferably may be a resistor.

According to another embodiment of the invention, it is possible to arrange the device in such a way that the impedance which is connected to the line L1, L2, with respect to magnitude and phase angle, is very much independent of the impedance which is connected to the collector of the transistor MT. An embodiment of that kind is illustrated in FIG. 2.

In the FIG. 2 embodiment the impedance Z3 comprises a transistor T3, the collector electrode of which is connected to one end of the impedance Z2 and the emitter electrode of which is connected to one pole of the storage condenser C1. The transistor T3 has a feed back circuit connected between the collector electrode on the base electrode, said feed back circuit comprising a resistor RT3 of the magnitude of 50 kilohms, and an RC-circuit which is connected in parallel with said resistor, said RC-circuit comprising a capacitor TC3 of the magnitude 1 pf. and a resistor R33, connected in series with said capacitor, of the magnitude of 20 kilohms. The base electrode of the transistor T3 is further connected to the ground conductor 0 through a resistor of the magniture l0 kilohms.

Still another transistor T is incorporated in this embodiment of the fork circuit. This transistor T has it emitter electrode connected to that pole of the storage condenser C2, which is not connected to the zero voltage conductor 0, through a resistor R12 of the magnitude of 1 kilohm. The collector electrode of the transistor T is connected to the base electrode of the transistor T3. The transistor Tf is of a conductivity type which is opposite to that of the transistor T3. In operation, a DC-bias is supplied to the base electrode of the transistor T3 through the transistor T1, so that the DC-voltage drop over the transistor T3 will be rather unnoticeable. One object of the resistor RT3, between the collector electrode of the transistor T3 and the base electrode of the said transistor, is, among others, to open the transistor T3 when the device is put into operation so that the charging of the storage condenser C1 may commence, as a result of which the transistor MT will open, and the storage condenser C2 commences to be charged.

Further, the base electrode of the transistor Tf is connected through a phase changing network Z to the conductor ML, through which signal voltage originating from the microphone M is conducted.

The transistor T3 now constitutes a load to the collector electrode of the transistor MT. Said load represents an impedance, the magnitude and phase angle of which is dependent of the design of the phase changing network 2 It is quite possible to design a network 2 so that, firstly said impedance will obtain a value which is suitable for balancing the line impedance in a favourable way, and secondly the frequency charcteristic or the signal voltages, which are supplied to the line L1, L2 through the microphone amplifier MF and the transistor MT, shall be the most favourable.

As seen from the line, the transistor T3 forms an mipedance, the magnitude and the phase angle of which is dependent only on the design of the feed back circuit TC3, R33, RT3. The components of this circuit have first to be chosen so that the wanted impedance, as seen from the line, is obtained (i.e., the impedance for the signal voltage coming from the line, said impedance being mainly resistive and inductive). Thereafter the phase changing network Zf has to be so adjusted, that the transistor T3, as seen from the collector, electrode of the transistor MT (that is for the signal voltage originating from the microphone M), will provide the wanted impedance (usually mainly resistive and capacitive). In spite of the fact that it is between the same points that the impedance Z3 acts (i.e., between the bottom end point of the impedance Z2 and one terminal of the storage condenser C1), said impedance Zf will obviously be different for voltages coming from the line and for voltages coming from the microphone.

The arrangement of an obvious impedance comprised in the fork circuit in a way indicated above may be used in connection with fork circuits other than the fork circuit according to FIG. 1. However it gives an especial advantage and is especially motivated in connection with the fork circuit according to FIG. 1. This is due to the fact that other fork circuits, without such steps, give the load on the line which is wanted, and also due to the fact, that the fork circuit according to the invention has a general design, that makes it especially suited for the arrangement of said obvious impedance.

It is important for the function of the device, that the transistor GT will be put into operation immediately after that the device has been taken in use and has been connected to the line L1, L2. To achieve this, the collector electrode of the transistor GT is supplied with current from the first storage condenser C1. Said condenser, which is rather small (it should have a capacitance value of about 25 pi), will be rapidly charged, and because of that, the transistor GT will immediately be supplied with current.

Because of the fact that the base electrode of the transistor MT is connected to the first storage condenser C1 through the resistor R7 and also because of the fact that said storage condenser, when the device is disconnected, very quickly will be discharged through the transistor GT, said transistor MT very quickly will be put out of operation when the device is taken out of use. Therefore, no self-oscillation of short duration will occur when the device is disconnected. Such a quick self-oscillation might otherwise give rise to a disturbing noise in the moment of disconnection, said noise being audible as a bump.

Still another advantage of the fork circuit according to the invention will arise according to the following: When a call is made in a normal way from the telephone instrument in which the fork circuit is incorporated, and the finger dial of the instrument is turned in the clockwise direction, a short circuit between the conductors L1 and L2 will take place, because of the contacts of the finger dial mechanism. Thereby, the transistor MT will be put out of operation by a time constant, which is determined by the capacitance of the storage condenser C1 and the impedance by the rest of the circuits which have influence on the voltages of the electrodes of the transistor MT, and further because of the difference between the charge voltages of the storage condensers C1 and C2. At this occasion no DC-current will flow through the rectifier bridge G, and, therefore, the line L1, L2 will be disconnected from the fork circuit for all signal voltages. Because the transistor MT is put out of operation very quickly, such signals which might have been transmitted from the microphone M to the loudspeaker H, will be attenuated so rapidly that a self-oscillation caused by acoustical feed back will not have sufiicient time to start.

In a practical embodiment of the invention the following values for the different components have proved to be suitable. R1=15 ohms, R2=10 ohms, R3=680 ohms, R4=10 ohms, R5:560 ohms, R6 10 kilohms, C1=25 ,uf., CZIIOOO pf.

What we claim is:

1. A hybrid circuit for electrically connecting a twoconductor signal transmission line to a transmitting device and to a receiving device, said hybrid circuit comprising an electron valve having three electrodes, means (a) for applying first signal voltages developed by said transmitting device to first and second ones of said electrodes with substantially equal phase and equal amplitude and (b) for supplying second signal voltages originating from said transmission line to said first and second electrodes with greater amplitudes than said first signal voltages, the voltage signal developed at the third one of said electrodes being controlled by the voltage applied between said first and second electrodes, and coupling means for applying the voltage signal developed at said third electrode to said receiving device.

2. The hybrid circuit defined in claim 1 wherein said electron valve comprises a transistor, wherein said first, second, and third electrodes are respectively defined by the base, emitter, and collector of said transistor, and wherein said line is connected through unequal impedances respectively to said base and said emitter.

3. The hybrid circuit defined in claim 1 wherein said electron valve comprises a first transistor, wherein said first, second, and third electrodes are respectively defined by the base, emitter, and collector of said transistor, wherein the conductors of said line are connected to separate terminals, with one terminal being connected to a source of reference potential and the other terminal being connected to one end of a voltage divider network, said hybrid circuit further comprising said voltage divider network which includes first, second, and third impedances and has its other end connected to said one terminal, the first impedance and said other terminal having a common junction, and said second impedance having common junctions with said first and third impedances respectively, means coupling said base to the junction between said first impedance and said other terminal, means coupling said transmitting device to the junction between said first and second impedances, and means coupling said emitter to the junction between said second and third impedances.

4. The hybrid circuit defined in claim 3 wherein said other end of said divider network is coupled to said source by a first capacitor, and wherein said base is connected to the junction between said capacitor and said divider network.

5. The hybrid circuit defined in claim 4 wherein said means coupling said transmitter device to the junction between said first and second impedances comprises a second transistor having its collector galvanically connected to said junction between said first and second impedances, its base coupled to said transmitting device to be supplied with signal voltages originating therefrom, and its emitter coupled to one terminal of a second capacitor, said second capacitor having its other terminal connected to said source and providing the bias applied to the base of said second transistor.

6. The hybrid circuit defined in claim 5 wherein said transmitting and receiving devices are respectively provided with first and second amplifiers, and wherein said second capacitor has an appreciably greater capacitance than said first capacitor and provides the operating current for said first and second amplifiers.

7. The hybrid circuit defined in claim 3 comprising first and second coupling networks, one of said impedances comprising a further electron valve having a control electrode supplied with control voltages originating (a) from said transmitting device and being coupled through said first network and (b) from said line and being coupled through said second network, the impedances of said first and second networks being so related that the apparent impedance of said further electron valve is different for the signal voltages respectively originating from said transmitting device and said line.

8. The hybrid circuit defined in claim 7 wherein the one of said impedances having said further electron valve is said third impedance, wherein said further electron valve comprises a second transistor, with said control electrode being defined by the base of said second transistor, the collector of said second transistor being connected to said second impedance and the emitter of said second transistor being coupled to said source through a capacitor, said second network providing a feedback loop connected between the base and collector of said second transistor, said first network including a separate transmission circuit for transmitting signal voltages from said transmitting device to the base of said second transistor.

9. The fork circuit defined in claim 8 wherein said second network is galvanically conducting.

10. The fork circuit defined in claim 8 wherein said separate transmission circuit comprises a third transistor of a conductivity type that is opposite to that of said second transistor, the collector-emitter path of said third References Cited UNITED STATES PATENTS 3,041,411 6/1962 Beatty 17981 3,075,045 1/1963 Clemency l791 KATHLEEN H. CLAFFY, Primary Examiner C. JIRAUCH, Assistant Examiner US. Cl. X.R. 179-170 Patent No.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Dated y l970 Column line 38,

Column 4, line 67,

Column line Column 7, line 12,

Column 7, line 14,

nwe Lindgren. Gunner Kiellander and Lennart Skoog It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

SIGNED AND SEAIEU oomissioner of Pa ants FORM PO-1050(10691 USCOMM-OC GOING-P69 e u s covtmmzm Panama ornc: mu 0-365-134 

